1. Field of the Invention
The present invention relates to a thermosensitive recording medium which utilizes a coloring reaction between an electron-donating coloring compound and an electron-accepting compound, and more particularly to a transparent thermosensitive recording medium which can be used as an image formation film for use with an overhead projector (OHP); an image formation film for use in the system of computer aided design (CAD); a film of a block copy, that is, an image formation film for gravure printing, offset printing, flexography and screen process printing; and a block copy film for textile printing.
2. Discussion of Background
There is widely known a thermosensitive recording medium which utilizes the coloring reaction between an electron-donating coloring compound (hereinafter referred to as a coloring agent) and an electron-accepting compound (hereinafter referred to as a color developer).
The application of the thermosensitive recording medium has expanded in recent years, and there is a demand for a transparent thermosensitive recording medium as an image formation film for the OHP, a second original for diazo copying, and a design drawing, and in addition, as a block copy film for gravure printing, offset printing and screen process printing.
For the above-mentioned applications, it is required that recording can be directly made on a thermosensitive recording medium by a thermal head. Such a thermosensitive recording medium is proposed in Japanese Laid-Open Patent Applications 61-121875 and 1-99873.
However, these thermosensitive recording media have the shortcomings that for the production thereof, there are required complicated steps such as microcapsulizing a coloring agent, dissolving a color developer in an organic solvent that is insoluble or slightly soluble in water, emulsifying the coloring agent in the form of microcapsules in the solution of the color developer to prepare a coating liquid for a thermosensitive recording layer, and coating the thus prepared coating emulsion dispersion on a transparent support. Further, the transparency of the recording media is not sufficient for use in practice. Therefore, when such conventional transparent thermosensitive recording medium is used as the block copy for printing, it is difficult to check the images formed on a plurality of block copies successively put one after the other because of insufficient transparency of each recording medium.
There is proposed in Japanese Laid-Open Patent Application 5-104859 a transparent thermosensitive recording medium which is excellent in transparency, and free from the above-mentioned problem with respect to the complicated producing method. According to this application, the first thing is to choose a solvent in which an organic acid to be employed as the color developer is slightly soluble or insoluble, and a coloring agent and a binder resin to be employed are soluble. A dispersion prepared by finely dispersing the organic acid serving as the color developer in the above-mentioned solvent, and a solution prepared by dissolving the coloring agent and the binder resin in the above-mentioned solvent are mixed and stirred, so that a coating liquid for a thermosensitive recording layer is obtained. The coating liquid thus obtained is coated on a transparent support, thereby obtaining a thermosensitive regarding layer. Then, a protective layer mainly comprising a resin is provided on the thermosensitive recording layer in order to fill up the minute surface roughness of the thermosensitive recording layer. By this method, a thermosensitive recording medium with high transparency can be obtained.
In recent years, however, in line with the trend toward high speed recording and economy of electric power, it is required to carry out thermal printing with the application of a low energy in light of the durability of a thermal head. In view of such a requirement, there is an increasing demand for a thermosensitive recording medium with high thermal sensitivity.
In Japanese Laid-Open Patent Application 8-175011, special attention is paid to the amount of solvent remaining in the thermosensitive recording layer of the obtained thermosensitive recording medium. According to this application, a thermosensitive recording sheet is produced in such a manner that one side or both sides of a base paper are coated with a coating liquid prepare by dissolving a resin in an organic solvent and the coating liquid is dried to prepare at least one resin layer on the base paper, and then a thermosensitive recording layer comprising a leuco dye and a color developer is provided on the above-prepared resin layer. It is reported that the fogging of background of the obtained thermosensitive recording sheet can be minimized during the storage by controlling the amount of organic solvent remaining in the thermosensitive recording sheet to 0.3 g/m.sup.2 or less, and therefore, the thermosensitive recording sheet can be used as a second original for diazo copying. The art disclosed in this application, however, does not aim to improve the thermal sensitivity of thermosensitive recording medium.
The addition of a plasticizer to the thermosensitive recording layer is proposed in Japanese Laid-Open Patent Application 9-216466, and the addition of an ultraviolet-curing resin thereto is proposed in Japanese Laid-open Patent Application 9-254553. When the plasticizer is singly added to the recording layer, there occurs migration in the recording layer, so that the blocking resistance of the thermosensitive recording medium is lowered. On the other hand, when only a liquid resin such as the ultraviolet-curing resin is added to the composition of the recording layer, the thermal sensitivity cannot be stably maintained.